1. Field of the Invention
The field of this invention relates to a transmitter circuit, a communication unit, and a method for amplifying a complex quadrature signal, and in particular a mechanism for amplifying a complex quadrature signal using a digital power amplifier.
2. Background of the Invention
In telecommunications, there has been a recent trend for device manufacturers to design wireless communication units that are capable of operating over multiple frequency bands, to enable the same device to operate in different geographical regions, as well as being able to switch between different service providers and different communication technologies. Hence, in the field of wireless (e.g., Radio Frequency (RF)) communication units, architectures for supporting communications across multiple and various frequencies have been developed, particularly in supporting high data rates that require wideband methodologies. A primary focus and application of the present invention is the field of RF power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems, particularly in supporting increasingly higher data rates, is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier, often referred to as linear transmitters.
In the field of linear transmitter techniques, digital power amplifiers (PAs), PA drivers and wireless local area network (WLAN) PAs are typically either IQ based transmitter designs or polar transmitter designs. It is known that digital polar transmitter designs can provide great efficiencies. However, the conversion from the digital (I, Q) signal to a RF version of the signal that exhibits amplitude modulation and phase modulation (AM, PM) tends to cause bandwidth expansion on amplitude modulation and phase modulation paths. However, it is also known that polar transmitter designs are impractical for wideband implementations, such as high modulation bandwidths supported by future communication standards, e.g. fourth generation (4G) standards.
A second known linear transmitter architecture is that of a digital quadrature (IQ) transmitter, as illustrated in FIG. 1. Digital IQ transmitter architectures support wide bandwidth modulation standards. However, digital IQ transmitter architectures introduce a significant efficiency loss due to quadrature power combining. In FIG. 1, there is illustrated an example of a simplified block diagram of part of a digital quadrature (IQ) power amplifier (PA) 100. The IQ PA 100 comprises a first (in-phase) array 115 of, say, switch-mode power cells 130 and a second (quadrature) array 120 of switch-mode power cells. The IQ PA 100 is configured to receive an IQ (In-phase/Quadrature) input signal comprising a first (In-phase) signal component 106 provided by first multiplexer (e.g. phase selector) 105 and a second (Quadrature) signal component 108 provided by a second multiplexer (e.g. phase selector) 110. The various signals can be selected based on the timing waveform of the ‘I’ and ‘Q’ signals in timing diagram 170. The ‘high’ portions of the respective waveforms identify the drive phases of the local oscillator inputs that may be selected for being applied to either DPA. The respective multiplexers 105, 110 are provided with independent local oscillator (radio frequency) inputs I-LO+, I-LO−, Q-LO+, Q-LO−, so that the ultimately produced signal can move between quadrants, with the sign bit on the LO signals dictating which axis is selected.
Quadrature control words I-BB[0 . . . 15] and Q-BB[0 . . . 15] are provided to the respective switch-mode ‘I’ and ‘Q’ power cell arrays 130 of the IQ PA 100 to select a number of power cell arrays 130 to be used. In this manner, the switch-mode power cells used for the ‘I’ digital PA (IDPA) and the switch-mode power cells used for the ‘Q’ DPA are independently driven, that is: an ‘I’ code controls a number of transistors turning on in the IDPA and a ‘Q’ code controls a number of transistors turning on in the QDPA. Thereafter, the two power amplified outputs of the respective quadrature radio frequency signals are combined in radio frequency combiner 140.
In this manner, the IQ PA 100 comprises a complex IQ based transmitter architecture that extends the digital domain through to the radio frequencies, thereby benefiting from the scalability and efficiency of digital components to a greater extent than conventional (linear) PA architectures. However, the combining of the two power amplified outputs of the respective quadrature radio frequency signals produces an undesirable efficiency loss.
Thus, there is a general need for concepts to better manage and reduce efficiency loss in digital transmitters.